1. Field of the Invention
This invention relates to a vehicle deceleration control device, and more specifically, to a vehicle deceleration control device that decelerates a vehicle through automatic braking force control.
2. Description of Prior Art
In the field of running control of a vehicle, such as automobile, it is known to execute deceleration control through which braking force or deceleration of a vehicle is automatically controlled based on a relative distance and a relative speed between the vehicle and its preceding vehicle or other objects in the front of the vehicle (referred to as a preceding vehicle etc. in the followings), ensuring a proper distance between the vehicle and its preceding vehicle, etc. Devices executing such deceleration control (or acceleration-and-deceleration control), for example, are shown in Japanese Patent Laid-Open Publications Nos. 2000-355232, 11-348746, 2002-67904, 10-32404, 10-297541 and 11-78600.
During automatic deceleration control as described above, a rapid speed change of a vehicle could deteriorate its driving and riding comfort. Thus, in order to prevent such a rapid change in a vehicle speed, JP 2000-355232 proposes to restrict the magnitude of a deceleration of the vehicle to a predetermined limit value. In this publication, because of the fact that the stronger braking force is required for ensuring a proper distance between vehicles as the vehicle speed increases, the limit value for the magnitude of a deceleration in the automatic control is also changed with the vehicle speed. In the other examples, JP 0.11-348746 and 2002-67904 disclose to limit the changing rate of a deceleration induced with automatic control, thereby suppressing rapid and/or frequent variation of a vehicle speed. In JP10-32404, 10-297541 and 11-78600, there are disclosed structures for suppressing a rapid change in braking and driving force at an end of automatic control owing to acceleration or deceleration (braking) actions of a driver during the execution of the automatic control.
By the way, even during deceleration control of an automatic control device as described above, a driver will execute a braking action (e.g., through depressing a brake pedal) to reduce the speed of his own vehicle at a deceleration higher than that provided by the automatic control when his vehicle becomes too close to a preceding vehicle, etc. Especially when a deceleration applied to a vehicle through automatic control is prevented from exceeding its limit value as in the devices of JP 2000-355232 and 11-348746, the deceleration or braking force applied through the automatic control is liable to be insufficient, increasing occasions in which a driver should judge by himself if still higher braking force is required. This is because the magnitude of an actually applied deceleration will be smaller than the magnitude of a target value of deceleration determined based on a relative distance and a relative speed between the own vehicle and the preceding vehicle, etc., i.e. the magnitude of deceleration required for ensuring a proper distance between the vehicles. Conventional vehicle deceleration control devices as shown in the above-mentioned publications however, rarely consider enabling a driver to judge the necessity of braking operation easily.
In more detail of this respect, for instance, in the device of JP 2000-355232, since a limit value for a deceleration is changed (reduced) together with reduction of the vehicle speed of its own vehicle through automatic deceleration control, it is difficult for a driver to estimate how much braking force will be applied through the automatic control. Due to this lowering of the restricted deceleration together with the reduction of the vehicle speed, a driver who experienced that high deceleration was allowed in accordance with a high vehicle speed at the time of the start of deceleration control may expect further deceleration through automatic braking, and thereby the starting of braking operation can be delayed. Especially when a large difference arises between the limit value for deceleration and a target value of deceleration determined based on a relative distance and a relative speed between a vehicle and an object in the front of the vehicle (a target deceleration) where the actual deceleration applied on the own vehicle becomes insufficient, the distance between the vehicles can be reduced unexpectedly for the driver.
Accordingly, conventional vehicle deceleration control devices may be improved for enabling a driver to judge the necessity of braking action more easily without inducing deterioration of driving and riding comfort.